1. Field of the Invention
This invention generally relates to an optical semiconductor element and an optical semiconductor device.
2. Description of the Related Art
Generally, a wavelength-tunable semiconductor laser has a gain for a laser emission and can select a wavelength of the laser. There are some methods of selecting a-wavelength. For example, the methods include a method of changing a resonant wavelength of loss or gain by changing a refractive index or angle of a diffractive grating or an etalon provided in a laser cavity. And the methods include a method of changing a resonant wavelength of the laser cavity by changing an optical length in the laser cavity (refractive index or a physical length of the laser cavity).
The method of changing the refractive index has an advantage in reliability or manufacturing cost, because a mechanical operating portion is not necessary being different from the method of changing the angle or length. The refractive index changing method includes changing a temperature of an optical waveguide, changing a carrier density in the optical waveguide by providing a current, and so on. A semiconductor laser having a Sampled Grating Distributed Reflector (SG-DR) is supposed as a wavelength tunable semiconductor laser that changes a temperature of an optical waveguide, where the SG-DR has a wavelength selection function.
In this semiconductor laser, if a reflection spectrum of a plurality of SG-DR regions (reflection region) is controlled preferably, a predetermined wavelength can be selected with a vernier effect. That is, this semiconductor laser emits a laser at a wavelength where reflection peaks of two SG-DR regions are overlapped with each other. It is therefore possible to select the lasing wavelength by controlling each of the reflection peaks of the SG-DR regions.
Generally, a heater is provided on a surface of one of the SG-DR regions. It is possible to change the temperature of an optical waveguide of the SG-DR region where the heater is provided, with heat generated by the heater. As a result, a refractive index of the optical waveguide is changed. Accordingly, it is possible to select a reflection peak wavelength of the SG-DR region where the heater is provided, by controlling the heating value of the heater.
And it is possible to control the lasing wavelength to be a desirable one by controlling the refractive index of the optical waveguide of the SG-DR segments, with use of a temperature control device providing heat to whole of a semiconductor laser. And so, Japanese Patent Application Publication No. 9-92934 (hereinafter referred to as Document 1) discloses a method of controlling a lasing wavelength of the semiconductor laser by providing an electrical power to a heater and a temperature control device separately.
However, it is apprehended that the temperature of an SG-DR region not having a heater is changed, when heating value of the heater is large. In this case, the lasing wavelength is off from a desirable one. In the semiconductor laser disclosed in Document 1, an active region (a gain region) and a DBR region (a reflecting region) having a heater are adjacent to each other. As is the case of the conventional art, if the heating value of the heater is large, the temperature of the active region not having the heater is changed. Therefore, the lasing wavelength is off from a desirable one in the semiconductor laser disclosed in Document 1.